The demand for rechargeable batteries has grown by leaps and bounds as the global demand for technological products such as cellular phones, laptop computers and other consumer electronic products has escalated. In addition, interest in rechargeable batteries has been fueled by current efforts to develop green technologies such as electrical-grid load leveling devices and electrically-powered vehicles, which are creating an immense potential market for rechargeable batteries with high energy densities and long calendar and cycle life.
Li-ion batteries are some of the most popular types of rechargeable batteries for portable electronics. Li-ion batteries offer high energy and power densities and slow loss of charge when not in use. In addition, they do not suffer from memory effects. Because of these benefits, Li-ion batteries have been used increasingly in defense, aerospace, back-up storage, and transportation applications.
Despite the push for better performance and lower cost in lithium ion batteries, there has been little change to the basic architecture of lithium ion cells, and, in particular, little change to the design of cell electrodes. A porous electrode active film has electrode active material particles and conductive particles bound together with polymer binder. This film is usually deposited onto a metallic current collector. Liquid electrolyte is soaked into the porous film. The pores ensure that there is a large surface area for charge transfer between the electrode active material and the liquid electrolyte.
Lithium-sulfur couples have been studied as they have the potential to produce batteries with higher capacity and higher energy than conventional Li-ion batteries. However, there are many problems with these systems. One problem is that sulfur is very soluble in typical liquid electrolytes. In a conventional sulfur-based electrochemical cell system, the sulfur in the cathode (in the form of polysulfides, for example) dissolves in the electrolyte and diffuses to the anode where it reacts with the lithium to form lithium sulfides. Trapped at the anode in the reduced state, the sulfur cannot be reoxidized to the original form and be returned to the cathode. This leads to rapid capacity fade and high impedance, resulting ultimately in cell death.
Another problem associated with lithium-sulfur systems arises from loss of surface area in the electrodes. During cycling, sulfur in the electrode region aggregates into larger particles, permanently changing the morphology of the cathode. The change in morphology results in reduced ionic and electronic conductivity. Thus it has not been possible to produce viable battery systems from lithium-sulfur couples.
It would be useful to construct a battery in which sulfur could be used as the active cathode material in order to exploit the high capacity and high energy that sulfur can provide.